W3C 'Valid' Logos
If your pages pass the appropriate W3C validation tests for HTML 4.01, XHTML 1.0 or CSS, you are invited to add one of these 'W3C logos to the page as a kind of 'badge of honour'.
One thing that confuses designers about validation is which standard they should be validating against. Well, it's like this. If you have a Doctype that says:-
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
That is what is determining the validity of your HTML against W3C HTML 4.01 Transitional standards. There is no reason why you shouldn't use another W3C standard. Validating against HTML 3.2
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"
is still an option and very useful if you suspect that a high proportion of your audience are still using older browsers. Some corporates still use Netscape 4.x as their 'supported' browser – hell, my own bank's online banking system still insists that I use Netscape 4.6, it refuses to load if I try to use Explorer 5.1 (Mac) and if it's anything more recent, forget it!
So, it is important that your markup validates to what you have in the Doctype. If you don't have a Doctype, the page might work perfectly but you can't call it 'valid' because you aren't comparing it against a particular standard. 'Valid' is a relative term, not an absolute one. You can also have a page that works perfectly in all current browsers, but doesn't pass W3C validation but as new browsers arrive, you will find that they are a lot less forgiving than older ones. Supporting deprecated features and sloppy markup makes browsers bigger and slower and that's why the latest generation of lean, mean browsers, like Safari, are more likely to choke on invalid pages.